The invention relates to the field of thermographic inspection. More particularly, the present invention relates to the uniform heating of large surfaces for thermographics inspection for defects in large structures.
Composite materials are being used to overwrap concrete columns and other concrete support structures in bridges and buildings for seismic reinforcement and/or structural rehabilitation. There are different manufacturing methods for applying composite overwraps. Each method has the potential for creating debonds at the composite overwrap to concrete interface. If a significant portion of the composite is debonded, the overwrap needs to be repaired or stripped off and reapplied.
An extensive survey of many nondestructive evaluation techniques has shown thermography to be a viable method for detecting debonds beneath composite overwraps. A thermographic inspection method typically involves two phases, a heating or cooling phase where a thermal gradient is induced through the thickness of the test article, and a monitoring phase where an infrared camera measures the test article surface temperature as the test article cools. Underlying defects, such as debonds and porous cavities, result in localized thermal conductivity deviations that give rise to hot or cold spots on the test article surface after the surface has been thermally treated. The Infrared camera can then be used to indicate the location, shape and approximate size of a surface region above each flaw. In some instances, the time required for the indication to develop can also be used to estimate the flaw depth. Infrared camera technology has progressed to the point that cameras of suitable thermal and spatial resolution are now in a package that is easily held by hand. However, the limiting factor for conveniently and quickly performing thermography often falls upon the method of establishing the thermal gradient. The thermal gradient can be established by various apparatus. It is important that the surface of the test article be uniformly warmed or cooled. It is particularly difficult to carry this out in a controlled and repeatable fashion when the test article surfaces are large. Single point heating sources such as flash lamps, spotlights and heat guns do not provide uniform heating across a wide area. Thermal blankets are cumbersome to manipulate and the application of hot water in a controlled fashion can be difficult. These and other disadvantages are solved or reduced using the invention.
An object of the invention is to provide an apparatus for uniformly heating a large surface through thermal radiation.
Another object of the invention is to provide a uniform rapid heating of a large surface through thermal radiation.
Still another object of the invention is to provide an apparatus that is easy to use by human beings for uniformly quickly heating a large surface through thermal radiation.
Yet another object of the invention is a method for thermographic detection of defects in large structures having large exterior surfaces heated by an apparatus that uniformly heats the large surfaces through thermal radiation.
The present invention is directed to an apparatus that radiates a uniform amount of thermal energy upon a large surface in a short period of time using manual manipulation. The heating apparatus is designed to be used as part of a thermographic inspection method in which an infrared imaging camera is used for thermographically inspecting large structures, such as large concrete structures overwrapped with composite. Although many of the effects of nonuniform heating can be removed through postprocessing of the thermal data, it is much more convenient and much less complicated if such postprocessing does not need to be performed. With acceptable unprocessed real time data, an operator can quickly outline the flaws or defects in the large structure at the time of the inspection, and repairs can be initiated immediately. To the greatest extent possible, the heating system should be hand deployable and manipulable. Handheld equipment is advantageous because anything that needs to be set on the ground or fixed to a structure must be adapted to the terrain around the area of inspection. The terrain can vary from pylons underneath piers to scaffolding hundreds of feet above the ground. There is also the consideration of setup time. Time spent setting up equipment only adds to the cost of the inspection. In the preferred form, the heating equipment heats a large surface with a uniform heat radiation pattern that can be conveniently applied by an operator manipulating the heating apparatus, and shortly thereafter, imaged by a hand held infrared camera for quickly detecting subsurface flaws. These and other advantages will become more apparent from the following detailed description of the preferred embodiment.